In the present state of the art of soybean culture in the United States it has become common practice to employ one of two or three available pre-emergent herbicides for weed control. In general this method gives relatively good control of grassy weeds. However, a number of broadleaf weeds benefit from the reduced competition from the grasses and create severe problems later in the growing season, sometimes sufficient to prevent the harvesting of the soybeans.
Although more effective, broader spectrum pre-emergent herbicides have been sought for use with soybeans, this approach to the problem has not been successful because of the sensitivity of soybeans to herbicides. An alternative method of controlling broadleaf weeds is to employ a post-emergent herbicide shortly after emergence of the weeds. This method is not yet a widely accepted practice because of a shortage of suitable herbicides. No more than two herbicides have given consistently acceptable results in the midwestern U.S. and cocklebur has only been controlled by applications that also caused substantial injury to soybeans. (See Successful Farming vol. 74 No. 4, page C12, February 1976.)
Just as soybeans are known to be sensitive to herbicides, peanuts are well known to be relatively insensitive, and permit the use of a much wider variety of herbicides. However, in the southeastern U.S. a herbicide-resistant weed has become a pest in peanuts in spite of the use of herbicides, probably benefiting from reduced competition. This weed, commonly called sicklepod has spread widely and is now probably the worst weed in soybeans in southeastern U.S. (See Weeds Today, Spring 1976, pages 12-14.) This weed problem is being attacked by means of mechanical cultivation and a variety of both pre-emergent and post-emergent herbicides. However, erratic results have been obtained with herbicides under the various soil and climatic conditions which exist in the area.